The use of mobile telecommunications networks has increased substantially over the two decades. Network operators of the mobile telecommunications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile telecommunications networks. The network operators of the mobile telecommunications network need to reduce the running costs of the base station as well as improve the coverage of the base station. One option to do this is to implement systems for relaying the telecommunications signals of the mobile communication network as a distributed antenna system (DAS).
The telecommunications standards typically provide a plurality of channels or frequency bands useable for an uplink communication from the handset to the radio station as well as for a downlink communication from the radio station to the handset.
For example, the communication standard “Global System for Mobile Telecommunications (GSM)” for mobile telecommunications uses different frequencies in different regions. In North America, GSM operates on the primary mobile communication bands 850 MHz and 1900 MHz. In Europe, Middle East and Asia most of the providers use 900 MHz and 1800 MHz bands.
The constantly increasing capacity demand in wireless telecommunications and the fact that about 80% of the traffic on the mobile communication system is generated indoors requires new methods to provide flexible signal relaying systems to enable an efficient spectrum usage. When indoor traffic is handled with a pure outdoor macro coverage solution, the signal penetration and the signal quality is poor in the indoor environment. Indoor coverage solutions with distributed antenna systems help overcome this issue, but the increasing capacity demands require more advanced indoor solutions beyond pure coverage systems
Active distributed antenna systems (DAS) or micro C-RAN have been developed to improve the coverage indoors. Theses systems have the capability of dynamic traffic/cell switching. The radio frequency (RF) signals in the DAS are communicated between a central hub and a plurality of remote units. The central hub is connected to one or more of the base stations.
In the DAS, the coverage of a single cell is not necessarily provided by a single one of the remote units. The term “cell” is used in the present disclosure according to the definition used for GSM and is equivalent to the definition of a sector in case of UMTS and LTE standards. The cell describes a single carrier or a multicarrier signal provided by a base station and which is typically relayed into a sector. The plurality of the remote units relay the same telecommunication signal of the cell throughout the coverage area of the cell. The coverage area of the cell is defined by the sum of the individual coverage areas of each ones of the remote units, which are assigned to the cell. In case a plurality of antennas is connected to at least one remote units the coverage area of the cell is the sum of the individual antenna coverage areas connected to the at least one remote unit assigned to the cell.
There is also a demand for sharing of the DAS between multiple mobile telecommunications network operators to reduce the costs for each individual network operator. Hence, the DAS needs to be able to efficiently combine the RF signals from the multiple network operators and to route the RF signals to one or more of the individual remote units. Ideally, the DAS has to deal with different requests from the different network operators regarding the cell structures and network design and optimization. Furthermore, the DAS needs to share power in the remote unit between the different network operators independently of the number of carriers used by each individual network operator.
The DAS may be used to provide coverage and capacity inside a building, as well as coverage and capacity in metropolitan or campus areas.
Document U.S. Pat. No. 7,761,093 B2 describes a method and system allowing multiple providers to share the same DAS. Each network operator's base station signal is digitized and can be routed to any digital remote unit at which the signal can be |[MW1] combined with any other signal from a different network operator within the same frequency band or another frequency band supported by the remote unit. Therefore, the digitally transmitted signals are then converted to analog RF signals in the remote unit and finally relayed into the coverage area of the remote unit.
The system described in document '093 provides for a full flexibility in assigning individual coverage areas of remote units or the sum of antenna coverage areas associated to a remote unit or portions of the system coverage area to different cells of the different network operators. However, this flexibility is only possible in full digital DAS systems. Full digital systems are expensive, as the digital systems require costly digital transceiver per remote unit, which are typically implemented per mobile radio frequency band.
U.S. Pat. No. 7,761,093 B2 also describes reassigning individual remote unit coverage areas to a different cell, for example for load balancing or network optimization. This is known as dynamic cell switching allowing. However, the system described in '093 cannot deal with inhomogeneous loads within the coverage area of one cell without changing the coverage area of the cell.
Furthermore, in a full digital DAS system the multiple signals from the different connected operator base stations need to be combined in order to relay the signals jointly into the cells. Multiple ones of the different operators typically own different spectral segments of the operating frequency band. Since these different spectral segments may comprise a single carriers or multiple carriers these different spectral segment can be considered to be carrier bundles in the following description. The operators are transmitting exclusively their carrier signals in the corresponding carrier bundles. Multi-operator DAS systems are expected to combine these different carrier bundles or the individual carrier signals owned by the operators prior to transmission via a common antenna that is common to all of the operators. This combination can either be done in an antenna combiner, as described in '093 (see FIG. 4) or the combination is implemented in the digital domain within the remote unit, as described in U.S. Pat. No. 8,682,338 B2.
The combining in the analog domain, after power amplification, as described in '093 will have an impact on power efficiency of the radio access node, due to the loss of the combiner. The digital combination of all carriers or carrier bundles of multiple operators within the remote unit as described in U.S. Pat. No. 8,682,338 B2 results in high digital signal processing complexity within each RU. The high costs for each remote unit will effect in total the cost of the system.
Hybrid systems comprising a digital sub-system and an analog sub-system have been proposed to reduce the costs. The hybrid system generally comprises a base station connected to a central hub either via its RF port or a digital port (e.g. CPRI, ORI, or digital interfaces like S1 for LTE or Iub/Iuh in case of UMTS, if the central hub comprises the corresponding base band signal processing unit). The RF signal is captured and digitized at the central hub and provided via a digital link to an expansion hub. The signal is converted in the expansion hub from the digital domain to the analog domain and further relayed to a plurality of the remote units. Document U.S. Pat. No. 8,428,510 describes an example of such hybrid systems. However, the system described in document U.S. Pat. No. 8,428,510 does not provide a solution for efficient routing for multiple network operators.